What is the event loop in JavaScript?

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The event loop is the mechanism that lets JavaScript — a single-threaded language — handle asynchronous work without blocking. It continuously checks whether the call stack is empty and, if it is, moves waiting callbacks into the stack for execution. The moving parts Call stack — where synchronous code executes, frame by frame. Web APIs / Node APIs — host-provided features like setTimeout , fetch , and I/O that run outside the JS engine. Task (macrotask) queue — callbacks for setTimeout , setInterval , I/O, and UI events. Microtask queue — callbacks for Promise.then , queueMicrotask , and MutationObserver . Event loop — the supervisor that pulls tasks into the stack in the right order. The loop's algorithm Run the current synchronous work to completion (drain the call stack). Drain all microtasks until the microtask queue is empty. Render updates if it is a browser environment. Pick exactly one task from the macrotask queue, push it onto the stack, and go back to step 1. Microtasks vs macrotasks Microtasks always run before the next macrotask, which is why Promise.then fires before a setTimeout(fn, 0) that was queued earlier. A long chain of microtasks can starve macrotasks and block rendering. console.log("1"); setTimeout(() => console.log("2"), 0); Promise.resolve().then(() => console.log("3")); console.log("4"); // Output: 1, 4, 3, 2 Non-blocking but not parallel Asynchronous APIs offload work to the host platform; the JS engine itself still runs on a single thread. Heavy CPU work inside a callback blocks the loop and freezes the UI — move such work to a Web Worker or a worker thread. Node's phased loop Node's event loop is organised into phases: timers , pending callbacks , poll , check (for setImmediate ), and close callbacks . Microtasks and process.nextTick run between every phase. This is why in Node the order of setTimeout and setImmediate can vary depending on context. The output order question (sync, microtask, macrotask) is the #1 event loop interview question. Key rule: ALL microtasks drain before the next macrotask. Promise.then is a microtask, setTimeout is a macrotask. Show you can predict execution order.